1. Field of the Invention
The present invention relates to an image processing method and apparatus, and radiation image photographing/processing apparatus that perform a blackening process on radiation image data representing a radiation image of a subject photographed using an exposure field aperture.
2. Description of the Related Art
It is practiced in the medical field to diagnose the presence of a disease or the progress of the disease by acquiring a radiation image of a subject as image data, displaying the radiation image on a display device, such as CRT or the like, based on the image data, and reading the displayed image to discover a lesion or observing the state of the lesion.
For that purpose, for example, X-ray mammography for breast cancer screening has been proposed. In the X-ray mammography, an image is generated by irradiating X-rays onto a breast, detecting X-rays transmitted through the breast by a cassette having therein a film, storage phosphor sheet, or flat panel detector, and reading out the detected radiation image as radiation image data. In the photographing for ordinary breast cancer screening, an 18×24 cm or 24×30 cm cassette is used, and close contact or magnification photographing is performed.
Then, where a suspected abnormal region is found in a breast, spot photographing is performed for the breast by locally compressing the breast with a pressing plate. Here, in order to acquire an X-ray image of the local area, the exposure area of radiation is limited by an exposure field aperture so that the X-rays are not irradiated other than the compressed area.
The spot photographing may generate an X-ray image of the portion of the breast on an area of a film or the like where the radiation is irradiated, but the other area does not receive radiation, whereby a so-called whited out image is generated, and when the entire radiation image is displayed, it becomes difficult for reading.
Consequently, Japanese Unexamined Patent Publication No. 2 (1990)-299640 (Patent Document 1) proposes to provide a detection means for detecting an X-ray exposure field area limited by an exposure field aperture and to perform a blackening process for increasing darkness in an area other than the X-ray exposure field area based on the detection result.
Performance of the blackening process only on an area other than the X-ray exposure field area as in the method described in Patent Document 1 has, for example, a problem, when the position of the film or the like in the cassette is displaced, that a discrepancy occurs between the position of the exposure field area detected in advance and the position of the exposure field area of the film or the like where the X-ray is actually irradiated, and a portion of a whited out area corresponding to the positional difference may possibly remain.
Further, the method performs the blackening process on the whited out area described above, so that if, for example, white pixel data are detected from the entire radiation image data, and the blackening process is performed on the detected white pixel data, then, for example, if a breast radiation image includes an image of a wire or the like attached to the breast, the pixel data of the image are also detected as white pixel data and the blackening process is performed thereon, causing another problem. In general photographing other than a breast, the radiation image sometimes includes a bone or the like, and the blackening process is performed on pixel data of the bone image, causing still another problem.
In view of the circumstances described above, it is an object of the present invention to provide an image processing method and apparatus, and radiation image photographing/processing apparatus capable of performing a blackening process on a whited out area without performing the blackening process on the inside of a radiation image of a subject and providing an easy-to-read radiation image.